Compact zoom lens

ABSTRACT

A zoom lens includes: a first lens group having a negative refractive power; a second lens group having a positive refractive power; a third lens group having a positive refractive power and including one piece of positive lens; and a fourth lens group having a negative refractive power, wherein the first, second, third, and fourth lens groups are sequentially disposed in an order from an object side to an image surface side of the zoom lens, all distances between adjacent lens groups change and the third lens group performs focusing while zooming from a wide angle position to a telephoto position, and a following condition is satisfied: 0&lt;(L 23W −L 23T )/L 23W &lt;0.5, wherein L 23W  denotes a distance between a lens surface of the image surface side of the second lens group and lens surface of the object side of the third lens group in the wide angle position, and L 23T  denotes a distance between a lens surface of the image surface side of the second lens group and a lens surface of the object side surface of the third lens group in the telephoto position.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the priority benefit of Korean PatentApplication No. 10-2010-0075674, filed on Aug. 5, 2010, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein in its entirety by reference.

BACKGROUND

1. Field of the Invention

Embodiments relate to a zoom lens suitable for an imaging deviceemployed in a miniature digital camera, a digital video camera, a mobilephone, a personal digital assistant (PDA), or the like.

2. Description of the Related Art

Recently, distribution of image forming optical devices, such as digitalcameras or digital camcoders that use an image pickup device like acharge coupled device (CCD) or a complementary metal-oxide semiconductor(CMOS), is rapidly expanding. Accordingly, a zoom lens that is highlyefficient, small, and light is required.

Also, an exchangeable lens camera, wherein a user may exchange lensesaccording to a photographing purpose, is typically required to haveexcellent optical performance with a compact size to be easily carried.

In order to manufacture a camera having a compact size, a driving unitsuch as a motor for auto focusing (AF), may be removed from a camerabody, and a function of the driving unit may be installed in anexchangeable lens. In this case, a size of the exchangeable lens isincreased and so as to counterbalance the increased size of theexchangeable lens due to the driving unit, it is important to design thelens itself to perform AF with a small driving source. Thus, an AF lensgroup is desired to be miniaturized and light-weighted in terms of anoptical design.

Generally, when a distance of an object changes from an infinity to ashort distance, AF is performed by using a first lens group that iscapable of obtaining an excellent optical performance in the entiredistance of the object. However, lenses included in the first lens grouptypically have large diameters and are heavy, and thus it is difficultto perform the AF by using a small driving source.

Even a conventional inner focus method, which does not use the firstlens group in performing AF, uses too many lenses in an AF opticalsystem, and thus it is difficult to maintain a low price and a compactsize of a camera.

SUMMARY

One or more embodiments include a standard zoom lens optical system,which realizes an excellent optical performance, has a compactstructure, and includes a small and light-weight auto focusing (AF) lensgroup.

According to an embodiment, a zoom lens includes: a first lens grouphaving a negative refractive power; a second lens group having apositive refractive power; a third lens group having a positiverefractive power and including one piece of positive lens; and a fourthlens group having a negative refractive power, wherein the first,second, third, and fourth lens groups are sequentially disposed in anorder from an object side to an image surface side of the zoom lens, alldistances between adjacent lens groups change and the third lens groupperforms focusing while zooming from a wide angle position to atelephoto position, and a following condition is satisfied:0<(L _(23W) −L _(23T))/L _(23W)<0.5,wherein L_(23W) denotes a distance between a lens surface of the imagesurface side of the second lens group and a lens surface of the objectside of the third lens group in the wide angle position, and L_(23T)denotes a distance between a lens surface of the image surface side ofthe second lens group and a lens surface of the object side of the thirdlens group in the telephoto position.

The Abbe number v3 of the positive lens included in the third lens groupmay satisfy v3>60.

The second lens group may include one piece of positive lens. At leastone surface of the positive lens included in the second lens group maybe aspheric.

The first lens group may include two pieces of negative lenses and onepiece of positive lens.

The first lens group may include one piece of negative lens and onepiece of positive lens. The first lens group may have at least oneaspheric surface. Both surfaces of the negative lens of the first lensgroup may be aspheric.

A distance between a lens surface closest to the image surface side ofthe second lens group and a lens surface closest to the object side ofthe third lens group may be uniformly maintained while zooming from thewide angle position to the telephoto position.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages will become more apparent bydescribing in detail exemplary embodiments with reference to theattached drawings in which:

FIG. 1 is a diagram of optical arrangements of a zoom lens in a wideangle position, a middle position, and a telephoto position, accordingto an embodiment;

FIGS. 2A, 2B, and 2C are aberration diagrams showing longitudinalspherical aberration, astigmatism, and distortion respectively in thewide angle position, the middle position, and the telephoto position ofthe zoom lens of FIG. 1, according to an embodiment;

FIG. 3 is a diagram of optical arrangements of a zoom lens in a wideangle position, a middle position, and a telephoto position, accordingto another embodiment;

FIGS. 4A, 4B, and 4C are aberration diagrams showing longitudinalspherical aberration, astigmatism, and distortion respectively in thewide angle position, the middle position, and the telephoto position ofthe zoom lens of FIG. 3, according to an embodiment;

FIG. 5 is a diagram of optical arrangements of a zoom lens in a wideangle position, a middle position, and a telephoto position, accordingto another embodiment; and

FIGS. 6A, 6B, and 6C are aberration diagrams showing longitudinalspherical aberration, astigmatism, and distortion respectively in thewide angle position, the middle position, and the telephoto position ofthe zoom lens of FIG. 5, according to an embodiment.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments will be described more fully withreference to the accompanying drawings. In the drawings, like referencenumerals denote like elements, and the sizes and thicknesses of eachelement may be exaggerated for clarity.

FIGS. 1, 3, and 5 are diagrams of optical arrangements of zoom lenses,according to embodiments.

Each of the zoom lenses according to the embodiments includes a firstlens group G1 having a negative refractive power, a second lens group G2having a positive refractive power, a third lens group G3 having apositive refractive power, and a fourth lens group G4 having a negativerefractive power, which are sequentially arranged in an order from anobject OBJ side to an image surface IMG side of the zoom lens. Anaperture ST is disposed on the image surface IMG side of the second lensgroup G2, and an infrared light blocking filter 500 is disposed betweenthe fourth lens group G4 and the image surface IMG.

All the first, second, third, and fourth lens groups G1, G2, G3, and G4move and all distances between adjacent lens groups change when zoomingfrom a wide angle position to a telephoto position. For example, thefirst lens group G1 moves with a locus convex toward the image surfaceIMG side, while the third through fourth lens groups G2, G3, and G4 movealmost in a straight line from the image surface IMG side to the objectOBJ side. All the distances between the adjacent lens groups change whenzooming, in the embodiments, and moving distances of the second andfourth lens groups G2 and G4 are identically maintained. In other words,a distance between a lens surface closest to the image surface IMG sideof the second lens group G2 and a lens surface closest to the object OBJside of the fourth lens group G4 is uniformly maintained, so as torealize a stable optical system with a low cost. When four (4) differentdriving units are used to mechanically change all distances between theadjacent lens groups, the structure of the zoom lens become complex,manufacturing costs of the zoom lens increase, and it is difficult toobtain a design performance while manufacturing the zoom lens due tooptical sensitivity. Accordingly, in the current embodiments, movingdistances of some lens groups are set to be the same while zooming,thereby structurally simplifying an optical system.

The third lens group G3 is an auto focusing (AF) lens group thatperforms focusing for image compensation according to a distance of anobject. The third lens group G3 is used as the AF lens group since ifthe first lens group G1 is used as an AF lens group like a conventionalzoom lens, a size of a driving unit of the first lens group G1 increasesbecause lenses included the first lens group G1 have largest externaldiameters and are heaviest from among lenses included in the zoom lens.Accordingly, the zoom lenses according to the embodiments use an innerfocus method. To reduce a weight of the zoom lens, the third lens groupG3 includes one piece of positive lens.

In order to miniaturize an optical system, each lens group may have astrong refractive power and a distance between lens groups may beminimized. In the wide angle position, the third lens group G3 may benear to the fourth lens group G4 so as to effectively compensate for aspherical aberration. In the telephoto position, a sufficient distancebetween the second lens group G2 and the third lens group G3 may besecured so that the third lens group G3 constituting the AF lens groupperforms focusing for compensating for an image according to a change ofan object distance from infinity to a short distance. Meanwhile, inorder to obtain a small zoom lens having a suitable zoom magnification,a main point of refractive power of an optical system from the secondlens group G2 to the fourth lens group G4 may be disposed on the objectside. Accordingly, the third lens group G3 may be disposed near to thesecond lens group G2.

In order to satisfy such contradicting conditions, an optimal distancebetween the second lens group G2 and the third lens group G3 may bedetermined. The zoom lenses according to the embodiments may satisfy afollowing condition.0<(L _(23W) −L _(23T))/L _(23W)<0.5

Here, L_(23W) denotes a distance between a lens surface of the imagesurface IMG side of the second lens group G2 and a lens surface of theobject OBJ side of the third lens group G3 in the wide angle position,and L_(23T) denotes a distance between a lens surface of the imagesurface IMG side of the second lens group G2 and a lens surface of theobject OBJ side of the third lens group G3 in the telephoto position.

The above condition determines a changing amount range of an air gapbetween the second lens group G2 and the third lens group G3 whilechanging the zoom lens from the wide angle position to the telephotoposition, with respect to an air gap distance between the second lensgroup G2 and the third lens group G3 in the wide angle position. Whenthe upper limit is exceeded, the moving distance of the third lens groupG3 is larger than the air gap distance, and thus it is difficult toobtain a sufficient space for focusing in the telephoto position. On theother hand, if the lower limit is not reached, a large air gap distanceis required to satisfy above-described requirements, and thus it isdifficult to miniaturize the optical system.

The zoom lenses according to the embodiments may also satisfy afollowing condition.v3>60

Here, v3 denotes an Abbe number of the positive lens included in thethird lens group G3.

The above condition is used to suppress longitudinal chromaticaberration that may be generated when an AF lens group includes onepiece of lens. A change of the longitudinal chromatic aberration isreduced by using a low dispersive lens. When the Abbe number is 60 orbelow, an optical performance deteriorates since the change of thelongitudinal chromatic aberration is increased while performing focusingaccording to a change of a distance of an object.

Detailed structures of first, second, third, and fourth lens groups G1,G2, G3, and G4 will now be described in detail with reference to lensdata.

A definition of an aspheric surface (ASP) in the embodiments is asfollows.

$x = {\frac{c^{\prime}y^{2}}{1 + \sqrt{1 - {\left( {K + 1} \right)c^{\prime 2}h^{2}}}} + {A\; h^{4}} + {Bh}^{6} + {Ch}^{8} + {Dh}^{10}}$

Here, x denotes a distance from a vertex of a lens to an optical axisdirection, y denotes a distance in a vertical direction with respect toan optical axis, K denotes a conic constant, A, B, C, and D denoteaspheric coefficients, and c′ denotes a reciprocal (1/R) of a radius ofcurvature in the vertex of the lens.

In the lens data, STO denotes an aperture and ASP denotes an asphericsurface. Also, EFL denotes an entire focal length, Fno denotes anF-number, 2ω denotes an angle of view, and D1, D2, D3, and D4 eachdenotes a variable distance between lenses.

FIG. 1 is a diagram of optical arrangements of a zoom lens in a wideangle position, a middle position, and a telephoto position, accordingto an embodiment. The first lens group G1 includes a first lens 111having a meniscus shape and constituting a negative lens, a second lens121 constituting a negative lens, and a third lens 131 constituting apositive lens. The second lens group G2 includes a fourth lens 211constituting one piece of positive lens. The second lens group G2generally includes two pieces of lenses in a conventional zoom lens, butso as to minimize the zoom lens, the second lens group G2 includes onepiece of positive lens in the current embodiment. Instead, at least onesurface of the fourth lens 211 is aspheric for aberration compensation.The third lens group G3 includes a fifth lens 311 constituting apositive lens, and the fourth lens group G4 includes a sixth lens 411constituting a negative lens and a seventh lens 421 constituting apositive lens.

Lens data of the zoom lens according to the current embodiment is asfollows.

EFL: 20.63~27.81~38.80 mm Fno: 3.65~4.16~4.92 2ω: 72.25~54.6~40.0 SurfRadius Thick Ind Abv 1 28.037 1.20 1.84666 23.8 2 13.541 3.63 3 −800.0001.20 1.75500 52.3 4 18.779 1.87 5 19.779 2.44 1.84666 23.8 6 78.483 D1 7Infinity 0.00  8* 21.051 2.16 1.58313 59.5 ASP K: −1.000000 A:−4.308475e−005 B: 2.409897e−008 C: −7.385826e−009 D: 1.077548e−010 9−32.698 0.10 STO Infinity D2 11  34.414 1.64 1.48749 70.4 12  −55.600 D313  −32.126 0.70 1.72825 28.3 14  17.616 1.12 15* −46.127 2.00 1.6935053.2 ASP K: 0.000000 A: 7.990710e−005 B: −7.283741e−007 C: 1.325045e−007D: −1.343725e−009 16* −13.481 D4 ASP K: 2.000000 A: 1.616460e−004 B:3.996546e−007 C: 9.450278e−008 D: 1.817467e−011 17  Infinity 2.791.51680 64.2 18  Infinity 0.53 IMG

Wide Angle Middle Telephoto Position Position Position D1 16.98 8.511.50 D2 5.46 4.87 3.88 D3 1.12 1.70 2.70 D4 27.66 32.97 40.42

FIGS. 2A, 2B, and 2C are aberration diagrams showing longitudinalspherical aberration, astigmatism, and distortion respectively in thewide angle position, the middle position, and the telephoto position ofthe zoom lens of FIG. 1, according to an embodiment.

In the spherical aberration, a line C having a wavelength of 656.28 nm,a line d having a wavelength of 587.56 nm, and a line F having awavelength of 486.13 nm are shown. In the astigmatism, T and S denotecurves respectively on a tangential surface and a sagittal surface.

FIG. 3 is a diagram of optical arrangements of a zoom lens in a wideangle position, a middle position, and a telephoto position, accordingto another embodiment. The first lens group G1 includes a first lens 112having a meniscus shape and constituting a negative lens, and a secondlens 122 constituting a positive lens. Unlike the zoom lens according tothe previous embodiment of FIG. 1, the first lens group G1 of the zoomlens according to the current embodiment of FIG. 3 includes two piecesof lenses, and at least one surface of the first lens group G1 isaspheric for aberration compensation. In the current embodiment, bothsurfaces of the first lens group G1 are aspheric. The second lens groupG2 includes a third lens 212 constituting one piece of positive lens,and at least one surface of the third lens 212 is aspheric foraberration compensation. The third lens group G3 includes a fourth lens312 constituting a positive lens. The fourth lens group G4 includes afifth lens 412 constituting a negative lens and a sixth lens 422constituting a positive lens.

Lens data of the zoom lens according to the current embodiment is asfollows.

EFL: 20.63~27.81~38.80 mm Fno: 3.65~4.16~4.92 2ω: 72.25~54.6~40.0 SurfRadius Thick Ind Abv 1 28.037 1.20 1.84666 23.8 2 13.541 3.63 3 −800.0001.20 1.75500 52.3 4 18.779 1.87 5 19.779 2.44 1.84666 23.8 6 78.483 D1 7Infinity 0.00  8* 21.051 2.16 1.58313 59.5 ASP K: −1.000000 A:−4.308475e−005 B: 2.409897e−008 C: −7.385826e−009 D: 1.077548e−010 9−32.698 0.10 STO Infinity D2 11  34.414 1.64 1.48749 70.4 12  −55.600 D313  −32.126 0.70 1.72825 28.3 14  17.616 1.12 15* −46.127 2.00 1.6935053.2 ASP K 0.000000 A: 7.990710e−005 B: −7.283741e−007 C: 1.325045e−007D: −1.343725e−009 16* −13.481 D4 ASP K: 2.000000 A: 1.616460e−004 B:3.996546e−007 C: 9.450278e−008 D: 1.817467e−011 17  Infinity 2.791.51680 64.2 18  Infinity 0.53 IMG

Wide Angle Middle Telephoto Position Position Position D1 18.95 9.41 1.5D2 3.80 3.62 3.12 D3 2.58 2.76 3.25 D4 27.2 32.40 39.58

FIGS. 4A, 4B, and 4C are aberration diagrams showing longitudinalspherical aberration, astigmatism, and distortion respectively in thewide angle position, the middle position, and the telephoto position ofthe zoom lens of FIG. 3, according to an embodiment.

FIG. 5 is a diagram of optical arrangements of a zoom lens in a wideangle position, a middle position, and a telephoto position, accordingto another embodiment.

The first lens group G1 includes a first lens 113 having a meniscusshape and constituting a negative lens, a second lens 123 constituting anegative lens, and a third lens 133 constituting a positive lens. Thesecond lens group G2 includes a fourth lens 213 constituting one pieceof positive lens. At least one surface of the fourth lens 213 isaspheric. The third lens group G3 includes a fifth lens 313 constitutinga positive lens. The fourth lens group G4 includes a sixth lens 413constituting a negative lens and a seventh lens 423 constituting apositive lens.

Lens data of the zoom lens according to the current embodiment is asfollows.

EFL: 19.00~27.81~40.00 mm Fno: 3.60~4.00~5.27 2ω: 76.98~54.28~39.54 SurfRadius Thick Ind Abv 1 22.890 1.20 1.88682 28.7 2 12.736 3.96 3 132.6521.20 1.75500 52.3 4 17.961 3.04 5 18.961 1.92 1.84666 23.8 6 32.174 D1 7Infinity 0.00  8* 19.237 2.25 1.59262 59.6 ASP K: −1.000000 A:−1.583792e−005 B: −1.807165e−007 C: 4.694510e−009 D: −6.548397e−011 9−35.909 0.89 STO Infinity D2 11  36.259 1.58 1.46562 88.0 12  −55.600 D313  −32.174 0.70 1.72427 27.9 14  20.432 1.38 15* −45.081 2.00 1.6935053.2 ASP K: 0.000000 A: −9.401767e−005 B: −9.801962e−006 C:3.166325e−007 D: −8.891901e−009 16* −14.063 D4 ASP K: 2.000000 A:5.553594e−005 B: −7.365590e−006 C: 2.489799e−007 D: −5.615594e−009 17 Infinity 2.79 1.51680 64.2 18  Infinity 0.55 IMG

Wide Angle Middle Telephoto Position Position Position D1 16.56 7.53 1.5D2 7.26 5.82 3.84 D3 1.12 2.56 4.54 D4 25.22 32.34 41.16

FIGS. 6A, 6B, and 6C are aberration diagrams showing longitudinalspherical aberration, astigmatism, and distortion respectively in thewide angle position, the middle position, and the telephoto position ofthe zoom lens of FIG. 5, according to an embodiment.

A following table shows that the zoom lenses according to theembodiments satisfy above-described conditions.

Zoom Lens Zoom Lens Zoom Lens of FIG. 1 of FIG. 3 of FIG. 5 (L_(23W) −L_(23T))/L_(23W) 0.29 0.17 0.42 V3 70.4 61.0 88.0

The zoom lenses according to the embodiments have a compact structure asan AF lens group is miniaturized and light-weighted, while havingexcellent optical performance.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

For the purposes of promoting an understanding of the principles of theinvention, reference has been made to the embodiments illustrated in thedrawings, and specific language has been used to describe theseembodiments. However, no limitation of the scope of the invention isintended by this specific language, and the invention should beconstrued to encompass all embodiments that would normally occur to oneof ordinary skill in the art. The terminology used herein is for thepurpose of describing the particular embodiments and is not intended tobe limiting of exemplary embodiments of the invention.

The use of any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. Numerous modifications and adaptations will bereadily apparent to those of ordinary skill in this art withoutdeparting from the spirit and scope of the invention as defined by thefollowing claims. Therefore, the scope of the invention is defined notby the detailed description of the invention but by the followingclaims, and all differences within the scope will be construed as beingincluded in the invention.

No item or component is essential to the practice of the inventionunless the element is specifically described as “essential” or“critical”. It will also be recognized that the terms “comprises,”“comprising,” “includes,” “including,” “has,” and “having,” as usedherein, are specifically intended to be read as open-ended terms of art.The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless the context clearly indicates otherwise. In addition, itshould be understood that although the terms “first,” “second,” etc. maybe used herein to describe various elements, these elements should notbe limited by these terms, which are only used to distinguish oneelement from another. Furthermore, recitation of ranges of values hereinare merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein.

What is claimed is:
 1. A zoom lens comprising: a first lens group havinga negative refractive power, and comprises two negative lenses and onepositive lens; a second lens group having a positive refractive power; athird lens group having a positive refractive power and comprising onepiece of positive lens; and a fourth lens group having a negativerefractive power, wherein the first, second, third, and fourth lensgroups are sequentially disposed in an order from an object side to animage surface side of the zoom lens, all distances between adjacent lensgroups change and the third lens group performs focusing while zoomingfrom a wide angle position to a telephoto position, and a followingcondition is satisfied:0<(L _(23W) −L _(23T))/L _(23W)<0.5, wherein L_(23W) denotes a distancebetween a lens surface of the image surface side of the second lensgroup and a lens surface of the object side of the third lens group inthe wide angle position, and L_(23T) denotes a distance between a lenssurface of the image surface side of the second lens group and a lenssurface of the object side of the third lens group in the telephotoposition.
 2. The zoom lens of claim 1, wherein the Abbe number v3 of thepositive lens included in the third lens group satisfies v3>60.
 3. Thezoom lens of claim 2, wherein the second lens group comprises onepositive lens.
 4. The zoom lens of claim 3, wherein at least one surfaceof the positive lens included in the second lens group is aspheric. 5.The zoom lens of claim 1, wherein the second lens group comprises onepositive lens.
 6. The zoom lens of claim 5, wherein at least one surfaceof the positive lens included in the second lens group is aspheric. 7.The zoom lens of claim 1, wherein the first lens group has at least oneaspheric surface.
 8. The zoom lens of claim 7, wherein both surfaces ofthe negative lens of the first lens group are aspheric.
 9. The zoom lensof claim 1, wherein a distance between a lens surface closest to theimage surface side of the second lens group and a lens surface closestto the object side of the fourth lens group is uniformly maintainedwhile zooming from the wide angle position to the telephoto position.10. The zoom lens of claim 1, wherein a distance between a lens surfaceclosest to the image surface side of the second lens group and a lenssurface closest to the object side of the fourth lens group is uniformlymaintained while zooming from the wide angle position to the telephotoposition.
 11. A zoom lens comprising: a first lens group having anegative refractive power, and comprises one negative lens and onepositive lens; a second lens group having a positive refractive power; athird lens group having a positive refractive power and comprising onepiece of positive lens; and a fourth lens group having a negativerefractive power, wherein the first, second, third, and fourth lensgroups are sequentially disposed in an order from an object side to animage surface side of the zoom lens, all distances between adjacent lensgroups change and the third lens group performs focusing while zoomingfrom a wide angle position to a telephoto position, and a followingcondition is satisfied:0<(L _(23W) −L _(23T))/L _(23W)<0.5, wherein L_(23W) denotes a distancebetween a lens surface of the image surface side of the second lensgroup and a lens surface of the object side of the third lens group inthe wide angle position, and L_(23T) denotes a distance between a lenssurface of the image surface side of the second lens group and a lenssurface of the object side of the third lens group in the telephotoposition.
 12. The zoom lens of claim 11, wherein the Abbe number v3 ofthe positive lens included in the third lens group satisfies v3>60. 13.The zoom lens of claim 12, wherein the second lens group comprises onepositive lens.
 14. The zoom lens of claim 13, wherein at least onesurface of the positive lens included in the second lens group isaspheric.
 15. The zoom lens of claim 11, wherein the second lens groupcomprises one positive lens.
 16. The zoom lens of claim 15, wherein atleast one surface of the positive lens included in the second lens groupis aspheric.
 17. The zoom lens of claim 11, wherein the first lens grouphas at least one aspheric surface.
 18. The zoom lens of claim 17,wherein both surfaces of the negative lens of the first lens group areaspheric.